1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof and more particularly to a semiconductor device at high integration degree and of high speed operation, and a manufacturing method thereof.
2. Description of the Related Art
In recent years, with a view point of increasing the integration degree and the operation speed of semiconductor devices, decrease in the inter-wiring capacitance has been required. Then, studies have been made vigorously for introducing dielectric films of lower dielectric constant than existent silicon oxide films (dielectric constant: 3.9-4.2) as inter-wiring dielectric films (hereinafter simply referred to as low dielectric constant films).
Typical low dielectric constant films of dielectric constant of 3 or less can include, for example, organic siloxane films, inorganic siloxane films and aromatic organic polymer films. The organic siloxane films contain at least methyl groups (xe2x80x94CH3). Methyl siloxane (MSQ) film and methylate hydrosiloxane (HMSQ) film are typical and methylate hydrosiloxane (HMSQ) film are typical examples. Further, inorganic siloxane films contain no methyl groups and hydrosiloxane (HSQ) films are typical examples.
Porous low dielectric constant films of further lower dielectric constant (dielectric constant; 2.5 or less) have also been studied and, particularly, porous organic siloxane films have often been studied because they are excellent, for example, in moisture resistance, chemical resistance, and mechanical strength.
However, the organic siloxane film involves a problem that adhesivity with a dielectric film at the upper layer is extremely low, which hinders practical application. Problems present in forming existent damascene copper wirings are to be described for the example of an organic siloxane film as an inter-level dielectric film (FIG. 1A to FIG. 2C).
At first, as shown in FIG. 1A, after forming a second inter-level dielectric film 7 on a substrate 21 including semiconductor devices and wirings, an organic siloxane film 8 is formed. The organic siloxane film 8 is generally formed by a spin-on coating method or a CVD (chemical vapor deposition) method. A silicon carbide film, a silicon oxide film or a silicon nitride film is formed to about several tens nanometers as a dielectric protection film 10 on the organic siloxane film 8. The dielectric protection film is formed with an aim of preventing the organic siloxane film 8 from degradation in the subsequent resist removing step by plasma ashing or polishing step for metal films. Successively, after forming a resist mask (not illustrated) on the dielectric protection film 9, first layer wiring trenches 11 are formed by dry etching as shown in FIG. 1B, and the resist mask is removed by plasma ashing. As shown in FIG. 1C, a titanium nitride film or a tantalum nitride film is formed thinly as a barrier metal film 12 and, further, a copper film 13 is formed. Finally, as shown in FIG. 2A, metals other than those in the first layer wiring trenches 11 are removed by a CMP (Chemical Mechanical Polishing) method to form conduction portions such as wirings or inter-level connection.
In the semiconductor device obtained by the procedures described above, since the organic siloxane film 8 of low dielectric constant is used as the inter-wiring dielectric film, the capacitance between wirings can be decreased effectively. However, adhesivity between the organic siloxane film 8 and the dielectric protection film 10 formed thereon is generally poor and delamination tends to occur during a CMP step or the like as shown at a delamination portion 22 in FIG. 2B. Such delamination results in deterioration of the reliability of wirings and lowering of the yield in the wiring steps.
For preventing the delamination failure, it has been studied a method of applying a plasma treatment just after the deposition of the organic siloxane film 8 to form an existent modified layer 23 on the surface thereby improving the adhesivity as shown in FIG. 2C. The modified layer is a layer formed by the plasma treatment on the surface of the organic siloxane film and having a carbon content lower than that of the organic siloxane film. For forming the existent modified layer 23, an oxygen gas has been used most generally as a plasma gas and water, ammonia, nitrogen, argon, hydrogen, helium or neon gas has also been known as other gas.
A method of improving the adhesivity by the plasma treatment using an active gas such as an oxygen gas or a gas mixture of ammonia and nitrogen has been disclosed in xe2x80x9cIntegration of Low k Methyl Silsesquioxane in a Non-Etchback/CMP Process for 0.25 xcexcm LSI Devicexe2x80x9d H. D. Joeng, et al., Proceedings of International Interconnect Technology Conference, 1999, pp. 190-192). At the surface of the modified layer formed by the plasma treatment using the active gas described above, the carbon content is decreased to about 1/5 of the carbon content in the organic siloxane film. As described above, since carbon causing that lowers the adhesion strength is decreased, film delamination can be prevented. However, since carbon is greatly decreased in the modified layer, the modified layer has high hygroscopicity. As a result, this results in a problem of increasing the dielectric constant and also increasing the effective capacitance between wirings. It is considered that a similar problem will occur even when water is used as an active gas.
As another example, a method of applying a plasma treatment to the surface of a non-siloxane type organic dielectric film by using an inert gas such as nitrogen, argon, hydrogen, helium or neon, there by improving the adhesivity to the dielectric film on the upper layer has been disclosed JP-A No. 106364/2000. Also in a case of applying the plasma treatment to the organic siloxane film using the inert gas, since the carbon content in the modified layer is decreased greatly, increase of the dielectric constant is inevitable.
Further, a further example of applying the plasma treatment on the surface of the organic siloxane film can include an etchback process. The etchback process is a process of forming a coated type organic siloxane film so as to fill a gap between metal wirings having unevenness and then conducting planarization by etching the organic siloxane film using a fluorocarbon type gas. Such a process is not effective for the improvement of the adhesivity. This is because the fluorocarbon type polymer deposits on the organic siloxane film to rather lower the adhesion strength. In the etchback process, an oxygen gas or an argon gas is usually irradiated for removing the gas fluorocarbon polymer, but the dielectric constant of the organic siloxane film is increased in this case.
A method of lowering the dielectric constant of the organic siloxane film increased by the plasma treatment is disclosed in JP-A No. 58536/2000. In this example, in the resist removing step using oxygen plasmas, the dielectric constant of the organic siloxane film exposed to the sidewall of the hole is increased. Then, the increased dielectric constant is lowered by a plasma treatment using one of a gas mixture of hydrogen and nitrogen, a fluorine gas, or a hexamethyl silazane gas. In this method, since a high reactive oxygen is irradiated to the organic siloxane film, the dielectric constant is increased greatly. Therefore, even if the dielectric constant is lowered by the plasma treatment of using the gas described above, the dielectric constant cannot be recovered to the same value as in the original organic siloxane film.
The present invention intends to provide a semiconductor device not increasing the dielectric constant of an organic siloxane film when the organic siloxane film is used for the inter-level insulating film, and improving the adhesivity between the organic siloxane film and the dielectric protection film while keeping the effective capacitance lower between the wirings, thereby avoiding the problem of delamination, as well as a manufacturing method thereof.
The foregoing problem can be overcome by forming an dielectric protection film after applying a plasma treatment using a fluorine-containing gas to the surface of the organic siloxane film.
According to this method, a modified layer of high adhesivity and low dielectric constant can be formed on the surface of the organic siloxane film.
The reason why the modified layer has high adhesivity relative to the dielectric protection film in the upper layer includes that the carbon content not lowers adhesivity compared with a not-modified organic siloxane film is decreased, unevenness on the surface is increased by the effect of the plasmas to increase the adhesion area and that active atoms on the surface are increased by the effect of the plasmas to increase chemical bond with atoms that constitute the dielectric protection film.
Further, the reason why the dielectric constant of the modified layer is low also includes that fluorine can prevent intrusion of moisture that causes increase of the dielectric constant and that etching is taken place simultaneously with the modification, so that a layer of high dielectric constant formed on the uppermost surface is removed. Accordingly, the modified layer has a dielectric constant about equal with that of the not modified organic siloxane film.
The modified layer of the invention is characterized by the atomic distribution in the direction of the depth of the organic siloxane film including the modified layer. The modified layer contains silicon, oxygen carbon, and fluorine. Further, the carbon content in the modified layer has such a distribution in the direction of the depth that the concentration is lowered as approaching the dielectric protection film. However, since the dielectric constant is increased if the carbon content is excessively lower, it is desired that the carbon content is not extremely lower compared with that of the organic siloxane film at the interface with the dielectric protection film. A practical carbon content that does not increase the dielectric constant is preferably 1/4 or more of the carbon content of the organic siloxane film at the interface with the dielectric protection film.
The organic siloxane film usable in the invention is a low dielectric constant film containing silicon, oxygen and carbon with the dielectric constant of 3 or less and it is preferred that 1/10 or more of silicon by element ratio is contained in order to obtain a practical mechanical strength. It is further preferred that 1/10 or more of carbon to silicon is contained with an aim of decreasing the dielectric constant and at the same time avoiding the moisture absorption. Further, with a view point of chemical stability, it is further preferred that carbon is contained in the form of methyl groups.
A plasma treatment gas that can be used in the invention is a fluorine-containing gas and a typical example can include NF3 (nitrogen trifluoride gas), SF6 (sulfur hexafluoride gas), F2 (fluorine gas), fluorocarbon gas (CxHyFz: x, y, z is each an arbitrary integer) as a single species gas or a gas mixture containing at least one of them. However, since a silicon polymer of high dielectric constant and of low adhesivity is deposited when the plasma treatment gas contains silicon, it is preferred that the gas for the plasma treatment does not contain silicon.
Further, since a fluorocarbon polymer film tends to be deposited on the surface of the modified layer when the carbon content in the gas for the plasma treatment is excessive, adhesivity between the modified layer and the dielectric protection film is lowered. Accordingly, a lower carbon content in the plasma treatment gas is preferred and, for obtaining practical adhesivity, the carbon content is preferably at 1/10 or less by atomic ratio based on the amount of fluorine. Further, most of plasma CVD apparatus are connected with NF3 gas as the cleaning gas and by the use of such an apparatus, plasma treatment, and formation of the dielectric film can be attained in one identical reaction chamber. Further, since NF3 gas contains no carbon, fluoro carbon polymer does not deposit on the surface of the modified layer. With the view points described above, it is particularly preferred to use NF3 as the plasma treatment gas.
Typical example of the dielectric protection film usable in the invention can include a silicon carbide (SiC) film, a silicon carbonitride (SiCN) film, a silicon oxide (SiO) film, a silicon oxynitride (SiON) film, a silicon oxycarbide (SiOC) film, a silicon nitride (SiN) film and an aluminum oxide (AlO) film as a single layer film or a stacked film comprising them in combination. In view of the adhesivity with the modified layer of the invention, silicon carbide is particularly excellent. The method of depositing the dielectric protection film can include a plasma CVD method, a thermal CVD method, a sputtering method, and a spin-on coating method with no particular restriction. Further, since the dielectric constant of the dielectric protection film described above is generally higher compared with that of the organic siloxane film, when the dielectric protection film is excessively thick, the capacitance between the wirings cannot be decreased effectively. Accordingly, the thickness of the dielectric protection film is preferably 100 nm or less.
Further, when the dielectric protection film is formed by an aromatic polymer film, high adhesivity can be obtained. For example, in the step of forming wirings, for example, by a dual damascene method, when the organic siloxane film is formed as the dielectric film of the inter-level connection layer, and then the modified layer of the invention is formed and an aromatic polymer film is formed as the dielectric film for the wiring layer, adhesivity between the modified layer and the aromatic polymer film can be improved.